Optical fibers, optical amplifiers and fiber lasers
KEY COMPONENTS FOR SPACE MISSIONS
Rare earth ion doped fibers (Er3+, Yb3+ or Er/Yb) are crucial elements in the design of optical amplifiers or fiber lasers. These components are then used in more complex systems such as fiber optic gyroscopes or high-speed free-space optical communications. Radiation tests show that commercial active optical fibers, components and associated systems are highly vulnerable to space radiations .
Deeper fundamental studies showed that their high sensitivity is rather explained by the nature of the aluminosilicate or phosphosilicate host glass matrices used to incorporate the ions than by the rare-earth ions themselves. Several techniques have been identified allowing to improve the radiation responses of these fibers and components.
Indeed, by co-doping the active fiber with cerium  or by loading it with hydrogen [2,3], it is possible to passivate the radiation induced defects relative to P or Al and thus improve the device radiation tolerance. Simultaneously, simulation tools have been developed to optimize the architecture of hardened systems with respect to the system profile of use in harsh environments . These hardening approaches by component or system do not affect the optical performances of the amplifiers as illustrated in Fig(1) that compares the characteristics of two amplifiers: one standard and one radiation-hardened.
Fig.1. Comparison between the performances pre-irradiation of Er/Yb optical amplifiers based on either a standard fiber or its cerium codoped counterpart.
Insert :illustration of the double-clad structure of these ErYb fibers .
Strong collaborations with CNES and Politecnico di Bari
This work is carried out in close collaboration with CNES, the French space agency, which is involved in the qualification of hardened optical fibers developed by LabH6 and the evaluation of the potential of complex architectures of fiber amplifiers and lasers.
Two co-supervisory (co-tutelle) PhD doctoral theses were carried out with the Politecnico di Bari Institute (Pr. Mescia, Italy) and allowed the establishment of multi-physical simulation tools to predict the radiative and thermal behaviors of optical amplifiers under irradiation
TOWARD THE VERY HIGH POWERS ...
Fig.2 and Fig.3 compare the performance of Er and Er/Yb amplifiers based on standard and hardened optical fibers. With the hardening solutions implemented by LabH6, it is today possible to design radiation-resistant systems up to doses of the order of 3 kGy(SiO2) (300 krad) covering the most demanding space missions. Today one of the main challenges concerns the development of systems delivering signals of very high power (> 10 W to 1550 nm). For these, it is necessary to study the radiation effects as well as the thermal contributions due to these high powers.
Studies combining simulations and experiments are under way (UJM / iXblue / CNES / Bari PhD thesis) to validate these computational codes before using them to design high power systems hardened to very high doses of radiation.
 S. Girard et al., J. Optics, 20 (2018) 093001
 B. Cadier et al., US20130101261A1 (2012)
 S. Girard et al., Opt. Express 20 (2012) 8457
 A. Ladaci et al., J. Appl. Phys. 121 (2017) 163104
 S. Girard et al., Opt. Lett., 39 (2014) 2541.
Optical amplifiers with gain losses of less than 10% at 1 kGy are available:
Fig.2. Performances of Er/Yb amplifiers based on a standard fiber (A # 1), Ce-codoped (A # 2) and their hydrogenated counterparts (A # 1H, A # 2H) .
Fig.3. Optical performances of Er amplifiers based on a standard fiber (A # 1), Ce-codoped (A # 2) and a hydrogenated version of the fiber Rad-Tol (A # 3) .